There has for a long time been an interest to make “more electric aircraft” as electric actuators are assumed to permit a lower total mass, less frequent maintenance, easier connections, i.e. cables should be used in place of hydraulic tubes, and possibly the total elimination of the infrastructure required for hydraulics.
One problem identified with electromechanical actuators is the possibility of “jamming”. An electromechanical actuator like that disclosed in the published International patent application No. WO 2004/034550 for Stridsberg Innovation AB/Lennart Stridsberg has a roller screw and nut and an electric motor connected to the roller nut. Certain types of catastrophic failures such as a broken roller nut roller may cause the roller screw to lock in the nut, making rotation impossible. If this happens to occur when the control surface is in an extreme position, the aircraft can become uncontrollable.
A related problem might occur if the electric motor would be completely disabled, i.e. incapable of creating torque but mechanically free to rotate. If this happens to occur when the control surface is in an extreme position, a large force will act on the roller screw. Roller screws are not self-retaining. Therefore, this force should translate into a torque acting on the roller nut to cause the roller nut and its associated motor to rotate, thereby moving the control surface towards a more neutral position. To ensure this, it is preferable to have a large lead angle roller screw as the efficiency from a linear motion of the screw to torque on the nut varies from some 0.9 on large lead angle screws to some 0.7 for fine lead angle screws. However, to reduce the mass of the electric motor acting on the roller nut, it is preferable to have a small lead angle roller screw.
Jamming is less likely to occur in a hydraulic actuator. If an actuator ceases to operate due to faults in pumps, valves or other energizing or control parts, a release valve can be opened that connects the two chambers on each side of the piston, thereby permitting oil to pass from one side to the other. However, hydraulic actuators may jam due to problems caused by deformations of the cylinder caused by enemy weapons or accidental collisions with external parts like the one that crashed a Concorde in Paris in July, 2000.
U.S. Pat. No. 6,208,923 for Hommel discloses a conventional fault-tolerant actuator control system having two parallel systems, each with a power supply, a process computer, motor power electronic circuits and a servo motor. The process computer in one of the parallel systems can only control the motor power electronic circuits in the same system, etc. The servo motors act on a common rack and there is one rack position sensor. One of the two systems are active and the other is shut down. The patented invention is to use the not active servo motor as an additional source of position information.
U.S. Pat. No. 5,670,856 for Le and Huggett discloses a fault tolerant system intended for aircraft control surfaces. The system has a common control computer, a common power supply and a common end actuator, for example a rack or a hydraulic cylinder, and three parallel systems each having one controller, one inverter (motor power electronic circuits), and one motor. The controller in one of the parallel systems can only control the motor power electronic circuits in the same system, etc. During a no fault condition, all three controllers, inverters and motors are active.
U.S. Pat. No. 6,402,259 for Corio discloses a wheel brake having redundant control and power supply. As shown in FIG. 6 of that patent, the system has three main power systems, two redundant BSCU computers that handle common tasks like anti-skidding control and four wheel brakes each including four servo motors. There are four redundant EMACs, each controlling two of four inner wheel servo motors and two of four outer wheel servo motors. As shown in FIG. 7, each EMAC contains four servo motor amplifiers, each of them connected to one motor that has an own tachometer, brake, gear train, actuator and position sensor. There is little risk of stability conflicts between the servo systems as the motors have no common mechanical stiff part. The closest mechanical connection is that their actuators press against the same brake disc.
U.S. Pat. No. 6,776,376 for Collins discloses an actuator system having two electro-mechanical actuators of equal size acting on a common control surface over a summing lever. The position of the control surface will be the average of the position of the two actuators. In the case where one actuator jams in for example a position 90% to the left, the control surface can be brought back to a neutral position by moving the other actuator to a position 90% to the right. The design will have the complexity of two actuators and a link arm. In case of jamming, the still operational actuator might be useless for any purpose but balancing the jammed actuator.
U.S. Pat. No. 6,237,433 for Rodrigues discloses an actuator having a roller nut that moves along an axially static roller screw. The roller nut is connected to an arm that is movable in parallel with the roller screw. A rather complex device including an extra motor, a gear box, a splined shaft longer that the total stroke of the actuator, an extra gear set, 18 rollers and a large diameter device similar to a roller nut permits the roller nut to be disconnected from the arm that is movable in parallel with the roller nut, thus permitting the control surface to move freely even if the roller nut/screw system has jammed. No damping system is disclosed.
U.S. Pat. No. 4,575,027 for Cronin discloses a rotational actuator that can rotate over a limited angle and that controls a control surface over a link arm. The stator of the actuator is assembled to the fuselage over bearings. A locking pin between the fuselage and the actuator stator will prevent the actuator stator to move relative to the fuselage. A solenoid can release the locking pin, thus permitting the control surface to move itself by turning the whole actuator. No damping system is disclosed.
U.S. Pat. No. 4,530,271 for Cronin discloses an actuator system including an electro-mechanical actuator connected to a control surface through a hydraulic device that basically is an electrohydraulic actuator without a pump. The intermediate device has the same stroke as the electromechanical actuator and transfers the same force as the electromechanical actuator. The arrangement is therefore almost as heavy as the combination of one electromechanical and one electrohydraulic actuator.
U.S. Pat. No. 4,179,944 for Conner discloses an actuator having two motors, one giving torque for ingoing and the other for outgoing movements. If the motors are commanded to give torque in the other direction, the motor rotors will slide along a tapped common shaft, approaching each other. This will release pins that normally connect the tapped shaft to a ball or roller nut. Release can be tested and then reversed. The design presupposes two actuators, one active and one passive, and therefore requires four motors for each controlled axis.
U.S. Pat. No. 4,858,491 discloses an actuator having two motors and a mechanical differentiating device that releases locking pins when the two motors move out of synchronization with each other.